In industrial piping, welding is often the preferred method for connecting industrial flanges due to the high integrity it provides. However, in low-pressure, utility, or highly explosive environments where welding is prohibited or impractical, the threaded flange (also known as a screwed flange) offers a convenient and cost-effective alternative.
This guide details the structure, key advantages, and operational limitations of the threaded flange, confirming its essential role among specialized flange types for certain non-critical applications.
Structure and Installation: No Welding Required
The defining characteristic of a threaded flange is the tapered pipe thread (internal helix) machined into its bore. This internal thread allows the flange to be simply screwed onto a pipe that has matching external threads, eliminating the need for complex, time-consuming, and expensive welding procedures.
Key Installation Benefits:
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Speed and Ease: Installation is fast, requiring minimal specialized equipment and labor.
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Non-Weld Zones: They are the ideal pipe fittings for areas where welding is dangerous due to the presence of flammable vapours or liquids, making them a safe choice in many explosive atmospheres.
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Temporary Systems: Their ease of assembly and disassembly makes them highly suitable for temporary piping, test rigs, or systems that require frequent maintenance and modification.
Performance and Limitations in Service
While the ease of connection is the major advantage, it is precisely the mechanical thread that limits the threaded flange’s pressure and temperature capabilities compared to welded alternatives like the Weld Neck or Slip-On.
1. Pressure Rating
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Low to Moderate Pressure: Threaded flanges are inherently weaker than butt-welded or fillet-welded connections. The material around the threads is reduced, and the mechanical joint is more susceptible to leakage under high pressure. They are generally restricted to low-pressure systems (e.g., ASME Class 150 or DIN PN16 and below).
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Sealing: For effective sealing, a thread sealant (such as a PTFE tape or pipe dope) must be applied to the threads to ensure a tight connection and prevent leakage of the transported medium.
2. Thermal and Vibration Sensitivity
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Vibration Risk: In systems subject to vibration, the mechanical joint of a screwed flange can loosen over time, leading to leaks.
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Thermal Expansion: Rapid thermal cycling or high-temperature service can cause differential expansion between the flange and the pipe material, placing stress on the threads and increasing the risk of joint failure or leakage. For high temperatures, welding is always the safer choice.
Ideal Applications for Threaded Flanges
Despite their limitations, threaded flanges are widely used in specific environments where welding is impractical:
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Utility Lines: Compressed air, inert gas, and non-critical low-pressure water systems.
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Small Pipe Diameters: They are most practical and economical for pipes with nominal diameters of NPS $4$ (DN $100$) and smaller.
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Galvanized Pipe: They are essential when using galvanized pipe fittings, as welding would destroy the protective zinc coating, negating the corrosion protection.
The selection of a threaded flange is a perfect example of prioritizing convenience and cost-effectiveness for non-critical flow where a small amount of internal turbulence from the threads is acceptable.
Deepen Your Flange Knowledge
Selecting the right connection type is a critical engineering decision based on pressure, fluid, and environment. Explore these related technical guides:
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Connection Comparison: Understand the structural advantages of welded joints:
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High-Pressure Solutions: Learn why the Weld Neck design is the standard for critical systems that cannot use screwed joints:
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Core Flange Resource: Return to our complete overview of all flange types and industrial flanges:
Do you have a project with explosion hazards where non-weld connections are mandatory? Contact us for specialized low-pressure joint solutions.
